1. Field of the Invention
The present invention relates to a process for preparing innovative binders which contain groups which react, with polymerization, with ethylenically unsaturated compounds on exposure to actinic radiation, and optionally also contain groups which are reactive towards isocyanates, and also to their use in coating compositions.
2. Description of the Prior Art
The curing of coating systems which carry activated double bonds by actinic radiation, such as UV light, IR radiation or else electron beams, is known and is established in industry. It is one of the most rapid curing methods in coating technology.
Particularly advantageous properties can be obtained if radiation curing is combined with an independently controllable second crosslinking step. Coating systems of that kind are referred to as dual-cure systems (e.g. Macromal. Symp. 187, 531-542, 2002).
Because of the environmental and economic requirements imposed on modern coating systems, that they should use as little organic solvents as possible, or none at all, for adjusting the viscosity, there is a desire to use coatings raw materials which are already of low viscosity. Known for this purpose for a long time have been polyisocyanates with an allophanate structure as are described, inter alia, in EP-A 0 682 012.
In industry these substances are prepared by reacting a monohydric or polyhydric alcohol with excess aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, EP-A 0 000 194 or EP-A 0 712 840). This is followed by removal of unreacted diisocyanate by means of distillation under reduced pressure. According to DE-A 198 60 041 this procedure can also be carried out with OH-functional compounds having activated double bonds, such as hydroxyalkyl acrylates, although difficulties occur in relation to the preparation of particularly low-monomer products. Since the distillation step has to take place at temperatures up to 135° C., in order to be able to lower the residue isocyanate content sufficiently (<0.5% by weight of residue monomer), it is possible for double bonds to react, with polymerization, under thermal initiation, even during the purification process, meaning that ideal products are no longer obtained.
EP-A 0 825 211 describes a process for synthesizing allophanate structures from oxadiazinetriones, although no radiation-curing derivatives having activated double bonds are known. All that is mentioned is the use of maleate- and/or fumarate-containing polyesters; the possibility of radiation curing is not described.
U.S. Pat. No. 5,777,024 describes the preparation of low-viscosity radiation-curing allophanates by reacting hydroxy-functional monomers which carry activated double bonds with NCO-containing allophanate-modified isocyanurates.
The formation of allophanate compounds by ring opening of uretdiones with alcohols is known as a crosslinking mechanism in powder coating materials (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419, and also U.S. Pat. No. 2003 0153 713). Nevertheless, the reaction temperatures required for this purpose are too high (≧120° C.) for the targeted preparation of radiation-curing monomers based on allophanate with activated double bonds.
Historically the direct reaction of uretdione rings with alcohols to allophanates was first investigated for solventborne, isocyanate-free, 2 K [2-component] polyurethane coating materials. Without catalysis this reaction is of no technical importance, owing to the low reaction rate (F. Schmitt, Angew. Makromol. Chem. (1989), 171, pp. 21-38). With appropriate catalysts, however, the crosslinking reaction between hexamethylene diisocyanate (HDI)-based uretdione curatives and polyols is said to begin at 60-80° C. (K. B. Chandalia; R. A Englebach; S. L. Goldstein; R. W. Good; S. H. Harris; M. J. Morgan; P. J. Whitman; R. T. Wojcik, Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium, (2001), pp. 77-89). The structure of these catalysts has not been published to date. Commercial products prepared by utilizing this reaction are also undisclosed to date.
In summary it may be stated that the preparation of low-viscosity radiation-curing allophanates by ring-opening reaction of alcohols with uretdiones at temperatures below 100° C. is unknown from the prior art.